CS 202 Fall 2013
Notes for Tuesday, September 17, 2013

Classes I Day 3 [13.11–13.12]

Private Member Functions

Member functions can be made either public or private. Why would you want a private member function? Usually, such a function is an internal-use utility function that simplifies the implementation of the class.

Consider the following class.

[C++]

class Bar {
public:
    void f1() const
    {
        for (int i = 0; i < 10; ++i)
        {
            cout << "ho" << endl;
        }
    }

    void f2() const
    {
        for (int i = 0; i < 20; ++i)
        {
            cout << "hi" << endl;
        }
    }

    void f3() const
    {
        for (int i = 0; i < 30; ++i)
        {
            cout << "hey" << endl;
        }
    }
};

The three member functions above are very similar. We might want to write a helper function that they all use. If we do not want the outside world to have access to this new functionality, then we would make the helper function private.

[C++]

class Bar {
public:
    void f1() const
    {
        printRepeat("ho", 10);
    }

    void f2() const
    {
        printRepeat("hi", 20);
    }

    void f3() const
    {
        printRepeat("hey", 30);
    }

private:
    void printRepeat(string msg, int num) const
    {
        for (int i = 0; i < num; ++i)
        {
            cout << msg << endl;
        }
    }
};

Objects in a Container

We can put objects in a container (for example, a vector or an array) just like any other type.

To start things off, here is a class.

[C++]

class Bar {
    ...
};

To make a vector of Bar objects, we could do something like this.

[C++]

vector<Bar> v;
Bar f;
v.push_back(f);
// And so on

Creating a container with a given size allows us to construct a large number of objects at once. However, we have little freedom regarding the parameters we pass to the objects’ constructors.

[C++]

vector<Bar> w(1000);

All 1000 objects above will be default-constructed. So we cannot make such a container unless class Bar has a default constructor. (Recall that a class’s default constructor is written for you as long as you declare no constructor. Otherwise—if you want to have a default constructor—you must write one yourself.)

Example Code

See fancyprint.h and fancyprint.cpp for the code for class FancyPrint, which, among other things, has a private member function.

See dofancyprinting.cpp for a C++ program that uses class FancyPrint. This program makes a container holding several objects.

For today’s lab work, see the 9/17 Challenge.